Magnetic resonance imaging (MRI) methods use the interaction between magnetic field and nuclear spins with the purpose of forming two-dimensional or three-dimensional images of a subject of interest, or at least an area thereof. These methods are widely used these days, notably in the field of medical diagnostics, since they do not require ionizing radiation and they are usually not invasive. MRI is used for example as imaging technique to visualize structural abnormalities of the body, e. g. tumor development.
An MRI apparatus uses a powerful magnetic field to align the magnetization of some atomic nuclei in the body, and radio frequency (RF) fields to systematically modify the alignment of this magnetization. This causes the nuclei to produce a rotating magnetic field detectable by a scanner, which constructs an image of the sampled area of the body. Magnetic field gradients cause nuclei at different locations to rotate at different speeds. By using gradients in different directions, 2D images or 3D volumes can be obtained in arbitrary orientation.
MRI in general requires that the subject of interest to be examined is arranged in a strong, uniform magnetic field B0, whose direction at the same time defines an axis, normally the z-axis, of the coordinate system on which the measurement is based. Temporal variations of the magnetization due to the applied RF fields can be detected by means of receiving RF antennas, which are configured and oriented within an examination volume of the MR device in such a manner that the temporal variation of the magnetization is measured in the direction vertically to the z-axis.
Spatial resolution in the body can be realized by switching magnetic field gradients. They extend along the three main axes and are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennas then comprises components of different frequencies which can be linked to different locations in the body/subject.
The signal data obtained by the receiving antennas matches to the spatial frequency domain and are called k-space data. The k-space data generally include multiple lines acquired with different phase encoding steps. Each line is digitized by collecting a number of samples. A set of samples of k-space data is transformed to an MR image, e. g. by inverse Fourier transformation.
Currently, there are two ways to perform MRI scanning, one with breath-hold and the other with free-breathing. In the first option, the subject of interest has to be ordered by an operator of the MRI system to stop breathing for the time of a scan. Free-breathing scans do not require interaction with the subject of interest, but take a longer time than breath-hold scans. Also image quality consistency is a potential issue in free-breathing scans. Hence it is desired to improve scan efficiency while preserving image quality and consistency.
In this context, document U.S. Pat. No. 4,878,499 A refers to a magnetic resonance imaging system. In this system, a static magnetic field is applied to a patient, and a gradient magnetic field and an excitation pulse signal are applied to the patient in accordance with a predetermined pulse sequence, so as to cause a magnetic resonance phenomenon in a selected slice of the patient. The magnetic resonance data of the magnetic resonance phenomenon is acquired, and the magnetic resonance image is obtained from the magnetic resonance data. The system has an announcement section for intermittently urging the patient to stop a body movement. In this system, a data acquisition section is operated under the control of a control section only while the patient stands still in response to the announcement of the announcement section, thereby intermittently acquiring magnetic resonance data in units of a predetermined volume.
Furthermore, document JP 2007 029250 A refers to a magnetic resonance imaging apparatus. A pulse sequence has a body movement detection sequence for detecting the body movement position of a subject and a photographing sequence for acquiring the image of the subject. A control means repeatedly executes the body movement detection sequence, a breathing stop instruction to the subject through a transmission means based on body movement information detected by the body movement detection sequence, the photographing sequence, and the instruction of breathing restart to the subject through the transmission means, also detects the body movement position by continuing the body movement detection sequence even after the breathing stop instruction, and the photographing sequence is executed when the body movement position is within a prescribed range.
Still further, document U.S. Pat. No. 5,363,844 A refers to an NMR system with a respiration monitor. The monitor provides a visual feedback to the patient which enables the patient to perform a series of breath-holds with the patient's diaphragm positioned at the same reference point. This enables NMR data to be acquired over a series of breath-holds without introducing blurring or image artifacts. Between breath-holds a navigator pulse sequence is used to gather NMR data from which diaphragm position is measured, and during each breathhold the pulse sequence is changed to gather NMR image data.
Document U.S. Pat. No. 6,144,874 A refers to a method for NMR image reconstruction. Data required to reconstruct an image is divided into central k-space views and peripheral k-space views. NMR navigator signals are acquired during a scan to indicate patient respiration and a first gating signal is produced when respiration is within a narrow acquisition window and a second gating signal is produced when respiration is within a wider acquisition window. Central k-space views are acquired when the first gating signal is produced and peripheral k-space views are acquired when the second gating signal is produced.
Article “robust abdominal imaging with incomplete breath-holds”, Nadine Gdaniec et al., Magnetic resonance in medicine vol. 71, no. 5, pages 1733-1742, ISSN 0740-3194, refers to breath-holding as an established strategy for reducing motion artifacts in abdominal imaging. A sampling pattern is designed to support image reconstruction from undersampled data acquired up to any point in time using compressed sensing and parallel imaging in combination with a navigator-based detection of the onset of respiration. It allows scan termination and thus reconstruction only from consistent data, which suppresses motion artifacts. The spatial resolution is restricted by a lower bound of the sampling density and is increased over the scan, to strike a compromise with the signal-to-noise ratio and undersampling artifacts for any breath hold duration.